1. Field of the Invention
This invention relates to a method for measuring the distance to a target by transmitting a frequency-modulated wave to the target and obtaining distance-pertaining information from a beat wave of a wave reflected from the target and the transmitted wave, and to a system suitable for use in the practice of the method.
2. Description of the Prior Art
Distance measurement methods or systems of the above-described sort, which are known generally as FM-CW radars, are employed to transmit FM-modulated microwaves to targets and to obtain distance-pertaining information contained in the phase fractions of waves received from the targets. To take out the distance-pertaining information from the phase fraction of each of such received waves, it is generally known to obtain a beat wave of the transmitted wave and received wave and then count the peaks of the beat wave. In this case, a count of n or n+1 is obtained over a single period of the FM modulation, depending on the state of the initial phase which is dependent on the distance to be measured. Thus, the above count is accompanied by a so-called intrinsic error d.sub.c. This intrinsic error is intolerably large when measuring short distances, because it reaches for example 3.75 m where a microwave of 10 GHz is employed and the maximum frequency shift of the FM modulation is set at 10 MHz. Since this intrinsic error is in inverse proportion to the maximum frequency shift, the intrinsic error may be reduced to a tolerable level provided that the maximum frequency shift is set at 1 GH.sub.z or greater. However, an apparatus or system capable of yielding such a high maximum frequency shift is extremely costly.
In order to reduce the above-mentioned intrinsic error, modulation is applied to the phase fraction of each beat wave and the number of peaks is counted over the period T.sub.1 of modulation. The period T.sub.1 of modulation for the phase faction is usually set at a level significantly longer than the period T.sub.2 of FM modulation, and moreover at a level an integer times the period T.sub.2 of FM modulation (namely, T.sub.1 /T.sub.2 =m). Accordingly, the following equation can be established by converting a count .SIGMA.N obtained over the period T.sub.1 of modulation into the number of peaks of a wave for the period T.sub.2 of FM-modulation: ##EQU1## where n.sub.0 is an integer, and 0.ltoreq..delta.&lt;1. Therefore, it is feasible to count fractions of the number of peaks.
The fractions may be correctly counted when, upon conducting the above-mentioned phase modulation, the phase is swept from .phi. to .phi.+k.pi.(k--being an integer) so as to render the resulting count irrelevant to the initial phase .phi. of each beat wave. If the modulation band of the phase reaches k'.pi.+.DELTA.(k': integer, 0.ltoreq..DELTA.&lt;.pi.), an error, however, develops in the counting of the fractions, leading to a so-called folded error.
To perform the above-mentioned phase modulation, it has heretofore been known to adopt the so-called double FM method or to provide a phase shifter with a high-frequency circuit unit.
In the case of the former method, a beat wave is represented by the following equation where the second FM-modulated wave is a triangular wave: EQU E'c=KA cos [.omega..sub.c .tau.+.gamma. cos .omega..sub..mu. t+8.pi..DELTA.F.sub.2 .tau..mu..sub.2 t]
where F.sub.2 : frequency of the second FM modulation, and ##EQU2## Thus, the distance-pertaining information is contained in the second member of the above equation, i.e., (8.pi..DELTA.F.sub.2 .multidot..tau..multidot..mu..sub.2 .multidot.t). The modulation band of a phase changes whenever the distance to a target varies. Hence, .DELTA.=0 is generally not satisfied except for the specific distance ndc. Here again, the above-mentioned folded error is developed.
This folded error reaches 70 cm or so where the frequency shift is set at 10 MHz and the prescribed distance is set at 5 m.
Incidentally, the double modulation method permits a reduction in the return error if the maximum frequency shift of the first FM modulation is set at a higher level. However, an apparatus or system relying upon frequency modulation of such a wide band lacks wide applicability and as mentioned above, is more costly.
In the case of the latter method, a modulator making use of a ferrite or PIN diode or a similar device is used as a phase shifter for electromagnetic waves. Such modulators are, however, accompanied by such shortcomings or poor linearity and thus cannot set modulation bands with acceptable accuracy. Therefore, use of the latter method results in difficulty in reducing measurement errors to desired levels.
As an alternative method, it has also been proposed to transmit a frequency-modulated electromagnetic wave of a wavelength .lambda. to a target and to receive a wave reflected from the target while moving a receiving antenna over a distance 1/4.lambda. or another distance which is an integer times the former distance. The received wave is mixed and detected together with the transmitted wave, thereby obtaining a beat wave of both waves. The thus-obtained beat wave contains distance-pertaining information and has been subjected to phase modulation owing to the above-mentioned movement of the antenna. The number of peaks of the beat wave is counted and averaged in accordance with the detection method of zero-cross points or the like, thereby obtaining information which represents the desired distance.
Furthermore, it has also been proposed to employ a mechanical phase modulation which makes use of a slide short method. The above-described antenna moving method and the slide short method are both mechanical modulation methods and thus require drive mechanisms to perform phase modulation. They are therefore accompanied by such drawbacks that systems or apparatus relying upon such methods are unavoidably high in price and large in size.
It has also been proposed to apply phase modulation indirectly to a beat wave by subjecting the carrier wave to phase modulation in each of the above-referred to methods. However, it has been proposed that a direct application of phase modulation to a beat wave having a low frequency is more advantageous. However, a beat wave is not a sine wave and its frequency varies in accordance with the distance to be measured. Therefore, it has been difficult to apply phase modulation directly to beat waves.
A variety of distance measurement apparatus or systems has heretofore been employed to carry out the above-described distance measurement methods. They may be used in various fields without physically contacting with targets. An example of such application is the non-contact measurement of the height or distribution pattern of a burden in a blast furnace.
With a view toward achieving stabilized operation of a blast furnace and an improvement in its fuel efficiency, it is generally practiced to control the distribution pattern of charged raw materials to a desired pattern in a blast furnace. For this purpose, it is required to measure the distribution pattern of each burden at a top portion of a blast furnace.
Many patent and utility model applications have been obtained on apparatuses or systems suitable for fulfilling the above purpose, namely, for measuring the distribution patterns of burdens charged in blast furnaces by means of electromagnetic waves such as microwaves.
The followings are examples of such patent and utility model applications:
Japanese Patent Laid-open No. 58560/1977:
A burden is scanned diametrically while rotating an antenna, so that the distribution pattern of the burden in a furnace can be measured.
Japanese Patent Laid-open No. 6669/1978:
A microwave distance-measuring apparatus is equipped with a measurement circuit and calibration circuit. The circuits are switched over so as to calibrate the apparatus.
Japanese Patent Laid-open No. 102073/1978:
A profile-measuring system is employed to determine the surface profile of a burden in a blast furnace. A microwave distance-measuring unit is arranged in a tip portion of a lance. The purging and cooling method for the distance-measuring unit is disclosed.
Japanese Utility Model Laid-open No. 151907/1981:
The angle of a reflect antenna is adjusted to cause a microwave to impinge upon the surface of a burden at a right angle.
Japanese Utility Model Laid-open No. 151958/1981:
A microwave transmission circuit unit is mounted in a tip portion of a lance, which unit is integrally constructed with antennas. Compensating waveguides are provided in the circuit unit so that loss of each microwave is cancelled within the circuit unit.
Japanese Patent Laid-open No. 59181/1982:
A calibrating transmission line is provided to calibrate each distance. The measurement mode and calibration mode are alternately and succesively switched over.
Japanese Patent Laid-open No. 166573/1982:
A microwave distance-measuring system making use of the AM modulation method is disclosed.
Conventional electromagnetic wave (hereinafter referred to as "microwave") profile meters or sounding meters are each constructed of an FM-CW radar which makes use of a microwave of either an X-band (10 GHz) or V-band (50 GHz) type. X-band microwaves are employed primarily for their possibility of manufacturing meters at lower overall costs. On the other hand, V-band microwaves are used principally for their capacity of achieving the focusing of beams by small antennas (spreading angle .alpha..lambda./D, where D is an antenna aperture and .lambda. is the wavelength of a microwave).
It should also be noted that the accuracy of measurement by a microwave profile meter or sounding meter is dependent on the reciprocal of the modulation band in the case of an FM-CW radar. Accordingly, frequency modulation bands of several GHz have been chosen for commercial FM-CW radars. As a result, their oscillators are high in price and large in size. On the other hand, it is better to construct an antenna and oscillator into a unitary assembly in order to make the radar smaller and facilitate its maintenace. For this purpose, a narrow-band oscillator may be used as the oscillator. Since a narrow-band oscillator can be fabricated using a Gann diode (GaAs device) so long as its modulation band is up to 150 MHz, a microwave radar has very compact dimensions and may thus be housed within a lance.
It is necessary to perform special signal processing in order to improve the accuracy of a narrow-band FM-CW radar such as those mentioned above. The above-listed Japanese patent and utility model publications may be referred to in this concern.